In recent years, under current circumstances in which environmental issues caused by waste plastics have been focused, biodegradable plastics, particularly aliphatic polyester resins and aliphatic-aromatic polyester-based resins, which are degraded after use into water and carbon dioxide by the action of a microorganism have drawn attention. Sheets, films, fibers, molded products and the like have been already put into commercialization of the products both domestically and abroad. Among plastic waste, however, foamed plastics which have been used for packaging containers, shock absorbers, cushioning materials and the like in large quantities have raised big social problems because of bulkiness, and thus solution thereof has been desired.
Therefore, researches on foamed plastics which exhibit biodegradability have been extensively conducted. Thus far, extruded foam of aliphatic polyester-based resins, mixed resins of starch and a plastic and the like have been developed, and started to be put into practical applications in part. Furthermore, also in the field of beads expansion in mold, generally referred to, in which foamed particles are produced once, and they are then filled in a mold followed by heating to obtain a foamed and molded product, the following techniques were developed with respect to the aliphatic polyester-based resin foamed particles.
According to Japanese Unexamined Patent Application Publication No. Hei 6-248106, a method has been known in which: aliphatic polyester particles obtained by increasing the molecular weight of an aliphatic polyester prepolymer through allowing to react with diisocyanate as a coupling agent are impregnated with a volatile foaming agent to give foamed particles; thereafter the particles are heated with heat water vapor to obtain preliminary expanded particles; and then they are placed in a mold and expanded while heating to obtain a molded product. This method raises problems of complicated steps and high cost due to involvement of the reaction step of diisocyanate, and further, only a product exhibiting a great percentage of contraction in molding could be obtained. Additionally, there is disclosed a technique of aliphatic polyester-based resin foamed particles with specified gel content by subjecting to a crosslinking treatment, and a molded product of the same (for example, see Japanese Unexamined Patent Application Publication No. Hei 10-324766), and the technique being excellent in formability with a lower molding percentage of contraction than that in the technique disclosed in Japanese Unexamined Patent Application Publication No. Hei 6-248106. However, since satisfactory product can be obtained in just a narrow range, improvement of the extent of gelation has been investigated (for example, see Japanese Unexamined Patent Application Publication No. 2001-49021, No. 2001-106821 and No. 2001-288294). These methods have enabled formation of a molded product of the foamed particles with low percentage of contraction in molding at a low density, however, addition of the crosslinking step further makes the steps more complicated, leading to deteriorated productivity and economical disadvantage. In addition, the molded products of the aliphatic polyester-based resin foamed particles obtained in such a manner were accompanied by problems of inferior product fusibility, and easy detachment at the interface of the foamed particles.
Meanwhile, JP-T (the term “JP-T” as used herein means a published Japanese translation of a PCT application) No. Hei 10-505620 also discloses a biodegradable aliphatic-aromatic polyester copolymer, and further defines a foamed and molded member as the molded product. However, this document does not clarify whether the foamed and molded member is an extruded foam or foamed beads, and still further, specific description for obtaining the foamed particles and molded product of the foamed particles at all. Moreover, in Japanese Unexamined Patent Application Publication No. Hei 10-324766, No. 2001-49021, No. 2001-106821 and No. 2001-288294, the aliphatic polyester-based resin is defined as a polymer including aliphatic ester bonds at a percentage of 60% by mole or more in the main chain (in Japanese Unexamined Patent Application Publication No. 2002-96323, No. 2002-121312 and No. 2002-187972, the percentage in the definition is referred to as 35% by mole), and a polyester resin which is poly(butylene adipate/terephthalate) obtained from aromatic dicarboxylic acid, aliphatic dicarboxylic acid, aliphatic diol as described in the aforementioned JP-T No. Hei 10-505620 is illustrated. Specific examples of the foamed particles prepared by the use of an aliphatic-aromatic polyester copolymer as a base resin are illustrated in Japanese Unexamined Patent Application Publication No. 2003-3001, and No. 2003-3002, however, almost all of these were studied with essential need of a crosslinking treatment. Thus, the foamed particles, their molded product, and method of producing the same were not substantially studied with respect to a polyester copolymer prepared without subjecting to, in particular, a crosslinking treatment. Additionally, descriptions about cell diameter are found in these documents, stating that when foamed particles having a cell diameter of equal to or greater than 50 μm, and preferably equal to or greater than 80 μm are not obtained, the molding is failed because the cell membrane is broken during the molding.
On the other hand, in connection with non-crosslinked and foamed particles and molded product of the foamed particles, the present applicant proposed the use of an ethylene-propylene random copolymer resin (for example, see Japanese Unexamined Patent Application Publication No. 2000-226466), the use of a poly(3HB-co-3HH)-based aliphatic polyester resin as a base resin (for example, see Japanese Unexamined Patent Application Publication No. 2000-319438), the use of an aliphatic-aromatic polyester copolymer as a base resin (for example, see Japanese Unexamined Patent Application Publication No. 2004-143269), and the like. In investigation of these polyester resin foamed particles and their molded product, cell diameter was not specifically studied. However, the polyester resin foamed particles disclosed in these documents had an occupancy rate of the cell diameter of equal to or less than 50 μm in an arbitrary two-dimensional cross section being less than 20%.
Thermal insulation property is included in characteristics as a foam, and it is important in energy saving of a variety of economic activities taking account of prevention of global warming. Conventional olefin-based molded product of the foamed particles had a comparatively high thermal conductivity of 0.041 W/mK or more (Comprehensive List of Foamed Plastics Techniques, 1989, Shin Takahashi, p. 158). Because a cell diameter-dependent suppressive effect of the radiant heat is involved in thermal conductivity, investigation of the cell diameter is important.